The invention relates to a process for combining transmitting/receiving devices of a cordless communication system to form a communicating unit.
The technical development of communication systems for the cordless transmission of voice and non-voice information is tied to various standards, analogously to the ISDN standard (Integrated Services Digital Network) which has existed for some time in line-connected communications. Apart from some national standards and several cross-boundary standards such as the CT1, CT1+ standard on analog basis and the CT2, CT3 standard on digital basis, a standard, the so-called DECT standard (Digital European Cordless Telecommunication; compare Nachrichtentechnik Elektronik, Berlin, Vol. 42, No. 1, 1-2/1992, pages 23 to 29, U. Pilger: "Struktur des DECT-Standards" (Structure of the DECT standard)), analogous to the global GSM standard (Groupe Speciale Mobile or Global Systems for Mobile Communication; compare lnformatik Spektrum, Springer Verlag Berlin, Vol. 14, No. 3, 1991, pages 137 to 152, A. Mann: "Der GSM-Standard--Grundlage fur digitale europaische Mobilfunknetze") (the GSM standard--basis for digital European mobile radio networks) for mobile radio, has been created on a European scale for the lower-power cordless communication between portables and a base station, with ranges of some 100 meters. It is an essential feature of the DECT standard that the base station can be connected to line-connected communication networks (e.g. PSTN=Public Switched Telephone Network; PTN=Private Telecommunication Network).
For the cordless communication according to the DECT standard, a dynamic channel selection of approximately 120 available channels is carried out.
The 120 channels result from the fact that in the DECT standard, ten frequency bands between 1.8 and 1.9 GHz are used, a time-division multiplex frame of 10 ms being used in time-division multiplex access (TDMA) in each frequency band according to the representation in FIG. 1. In this time-division multiplex frame, 24 time channels (from 0 to 23) are defined which provides a frame structure. This frame structure is then used in such a manner that for each frequency band, 12 stations of a DECT system can operate simultaneously in duplex mode. A time slot of in each case 417 .mu.s is allocated to the 24 time channels. This time slot specifies the time in which information (data) are transmitted. This type of transmitting information in duplex mode is also called the ping-pong method because transmission takes place at a particular time and reception takes place at another time. In this ping-pong method, one time frame or pulse (burst) of 365 .mu.ts is transmitted in each time slot, which approximately corresponds to a frame length of 420 bits. The succession in time of the transmitted pulses defines a channel, the so-called physical channel, with a data throughput of 42 kbit/s for the time slot, 6 kbit/s for a guard space used in order to avoid overlaps by adjoining time slots, and 1.152 Mbit/s for the time-division multiplex frame. In the DECT standard, the time frame shown in FIG. 2 is allocated to a physical layer (PH-L) and is frequently also called the D field.
In addition, a number of layers are defined in the DECT standard, analogously to the ISDN standard with the ISO/OSI 7-layer model. One of these layers is the physical layer (PH-L). Another layer is a Medium Access Control Layer (MAC-L) which is allocated an A field and a B field according to FIG. 3. The A field comprises 64 bits which, inter alia, are used for messages when combining the portables and base station of the DECT communication system. The B field comprises 324 bits, 320 bits of which are used for voice dam and 4 bits for detecting partial interferences of the pulse.
In its simplest form, the DECT communication system has a base station with at least one mobile station. More complex (e.g. networked) systems contain several base stations having in each case several mobile stations. Due to the 24 time channels defined in the DECT standard, up to 12 mobile stations can be allocated to the base station, which communicate with the base station in duplex mode. For the time-division multiplex frame of 10 ms, also defined in the DECT standard, duplex mode means that information is transmitted every 5 ms from the base station to a mobile station or conversely. So that the base station and mobile station can communicate with one another, it must first be ensured that the two stations (communication partners) are combined to form a communicating unit.
For this combining, a booking and registration procedure is carried out according to European reference EP-A2-0 301 573 (corresponding to U.S. Pat. No. 4,864,599). In this known booking and registration procedure, the combining between a base station and a mobile part is achieved as follows:
(1) Transmitting of a product-specific identification word from the base station to the mobile part which is ready for booking, PA1 (2) Checking of the received identification word in the mobile part (determining the correspondence with an identification word stored in the mobile part and subsequent acknowledgement of the agreement found), PA1 (3) Transmitting of a system- and telephone-specific identification code from the base station, PA1 (4) Storing of the identification code in a memory of the mobile part. PA1 (1) of the supplementary message with the reference information stored in the second transmitting/receiving device, the second transmitting/receiving device is synchronized with the first transmitting/receiving device, PA1 (2) of the legitimation message with the reference
In spite of this known procedure used when combining base and mobile stations to form a communicating unit, it is still possible for outside users of a cordless communication system to be able to conduct unauthorized telephone calls with the aid of product-identical (identical product-specific code word) mobile stations via the base station of the cordless communication system when this is connected, for example, to a line-connected communication network.